1. Field of the Invention
The present invention relates to a laser welding method which increases the suppression of defective welding.
2. Description of the Related Art
Laser welding is suitable for precise and fine processing characterized by deep fusion and less thermal distortion.
Beam irradiation onto an object to be welded in the laser welding is carried out by the following methods.
(i) A beam fixing method in which the irradiation point of a laser beam is fixed to one point and an object to be welded is moved in order to be welded.
(ii) A spinning method in which a beam is spun while being fed.
(iii) A scanning method in which a beam is moved in zigzags.
It is important in laser welding to reduce defective welding such as porosity (pores) in the welded portion. If the welded portion has porosity, the strength of the welded joint weakens, and reliability of welding connection is lowered.
However, in the beam fixing method mentioned above (i), the welded portion easily contains, porosity. Depending on the material of the object to be welded, assisting gas is involved, and high porosity occurs, and therefore, a satisfactorily welded joint cannot be obtained. Also, this method (i) has the drawback in that the gap width, which can be welded in butt-welding, is narrow, or the joint strength obtained by lap welding is weak.
Therefore, as a countermeasure for the method (i), the abovementioned spinning method of (ii) and scanning method (iii) are considered.
In the spinning method, the beam is moved a certain number of times across the portion welded once of an object to be welded, and the resultant stirring effect in a fusing pool of the heat-fused object promotes discharge of the gas products. This method is effective to suppress porosity, if the spinning diameter and spinning frequency of the beam and feed speed of the object to be welded are selected properly. However, since the beam track between the left and right of the welded line is different, the welded condition may differ between the left and right of the butted point, and therefore, use of the method is restricted because of difficulty in setting the proper conditions. Also, even if this method can be used, the effect of suppressing porosity is limited, and a higher result cannot be expected.
On the other hand, in the scanning method, although the welded condition can be made uniform, the effect of suppressing porosity cannot be obtained.
Also, a method has been developed in which reflecting mirrors provided in the path of the laser beam are vibrated by a galvanometer to vibrate the focus position of the laser beam within a plane perpendicular to the optical axis (xe2x80x9cPresent Status and Themes of Laser Beam Weaverxe2x80x9d by Shigeki Fujinaga, et al, pp43, xe2x80x9cMonograph Collection of 26th Society for Study of Laser Heat Processing 1991xe2x80x9d). This method is developed for the purpose of improvement of the welding strength by expanding the fusing width, whereby the spinning method and scanning method can be selected, however, the effect of increased suppression of porosity over the spinning method is not achieved.
Therefore, the object of the invention is to provide a laser welding method whereby an effect of suppressing porosity can be obtained regardless of the feed speed of the object to be welded and the beam track, and the advantages in both the spinning method and scanning method can be obtained by combining them.
In order to achieve the above theme, in the invention, a laser beam as a heat source is irradiated onto an object to be welded while the focus position of the beam is vibrated in the optical axis direction (the direction parallel to the optical axis, that is, the direction of the depth of the welded portion) to carry out welding.
Also, a method is employed as necessary in which the laser beam is irradiated onto the object to be welded while the focus position of the beam is vibrated in two directions, that is, the optical axis direction and a direction perpendicular to the optical axis.
When the focus position of the laser beam is vibrated in the optical axis direction, the fusing pool formed by heat-fusion of the object to be welded is efficiently stirred by the laser beam. As a result gas containing in a keyhole at the center of the fusing pool, which is a cause of porosity, evaporates easily to the open air, whereby porosity is suppressed.
This method is not restricted in use since the fusing pool can be stirred without spinning the beam, and can bring an effect of suppressing porosity in all laser welding.
Also, this method can be combined with the spinning method to increase the effect of suppressing porosity, or combined with the scanning method to suppress porosity while making the welding condition uniform.